This invention relates generally to electromagnetic actuators and more particularly to such actuators of a "no work" type in which the armature or actuator is restrained against a biasing force and selectively released to an extended position.
In certain applications such as serial printers, magnetic actuators are used to operate print hammers and employ bucking coils to oppose the hammer retention force and release or fire the hammer. In the known devices, the release current for the bucking coil is relatively substantial so that separate components are used to construct the current drivers for the coil. These components add a significant cost to the alternative of manufacturing the drivers as integrated circuits, as are most of the printer control circuits.
As printer technology is advanced, the hammer actuators are required to be capable of higher repetition rates which result in problems of dissipating the coil heat caused by the greater proportion of on time per unit of time. Efforts have been made to advance the technology be reducing hammer mass, using supplemental windings and reset devices for the actuators.
One method of varying the magnitude of the release current is discussed in the aforementioned Helinski patent application, Ser. No. 974,297 in which a permanent magnet serves as the center leg of the three-legged core to provide a pair of parallel flux paths with the actuator serving as a component in one path and a variable reluctance element forming a part of the second path. As the reluctance of the latter path varies, it affects the flux density of the first path and permits energization of the bucking coil at a time when the actuator holding force is less than maximum. This structure, however, has a disadvantage of having to also overcome the reverse magnetomotive force due to a flux undulation in the bucking coil. A further disadvantage is evident in the use of multiple core structures with a common magnet and bucking coil in that the core structures are influenced by the change in reluctance of adjacent cores and thereby the flux passing through the coil.